"These particles are so small that they are almost impossible to recover," he says. It is only possible using costly processes like heating to high temperatures or ultra-high-speed centrifugation.

Light switch

But now Eastoe and Bristol colleague Charl Faul have identified a low-cost alternative that can easily recall nanoparticles when its time for them to stop work.

Under visible light the remote-control chemical acts as a mediator that smoothes the interface between usually incompatible liquids and solids.

The substance can make water and oil mix, by coating nanometre-sized water droplets that become spread evenly through the oil. It can also keep nanoparticles from clumping together, so they spread widely through a liquid.

Illumination with UV light, however, turns the mediating chemical into a partisan, preferring to dissolve in watery liquids. That instantly frees up dispersed water droplets to group together into a separate layer from oil. Similarly, the UV version of the chemical lets nanoparticles clump together into easily collected lumps (see video, top).

Reversible reaction

The chemical is a modified form of a surfactant or wetting agent commonly used in engines. The researchers modified it by inserting azobenzene, a kind of chemical switch that takes on different shapes when illuminated with visible or ultraviolet light.

The visible light conformation is equally compatible with watery and oily liquids, while the UV light version is more soluble in water. The researchers found that they could mix and separate oil and water using only a tiny quantity of the new chemical diluted in normal surfactant.

"The beauty of the system is it's a reversible reaction," says Eastoe. A smooth emulsion of nanoparticles can be converted back into a separated system at the flick of a UV light switch, and at a fraction of the cost of a high-energy centrifuge.

In 2006, Philip Jessop's team at Queen's University in Kingston, Canada, produced their own reversible surfactant to mix and separate oil and water on demand. Their chemical is activated by bubbling carbon dioxide through a solution, and deactivated with a trickle of everyday air.

Jessop says that his approach is more useful for dealing with opaque emulsions, such as those created by crude oil spills, where light can't penetrate very deeply. But light does have advantages in transparent emulsions. "The overwhelming advantage of this design is the lack of chemical reactivity in the switching group," he says. Traditionally, switching systems use highly reactive chemical groups that are incompatible with some mixtures.

But he says the study is interesting primarily because it shows that a tiny quantity of switchable surfactant is all that's needed to tip the balance towards either an even mix or two separate solutions. "That is great news for the example application, and if it can be shown to have wider applicability, great news for switching chemistry in general."

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